Last year, for instance, the blog Perfect Price surveyed the six states in the country with the most dispensaries: Arizona, California, Colorado, Michigan, Oregon and Washington. Oregon's price per ounce was then the cheapest, at $214, and of the major cities in those states, five of the eight cheapest were in Oregon.

Mark Pettinger, spokesman for the Oregon Liquor Control Commission, which regulates recreational marijuana in the state, says Oregon's prices are low by design.

"The whole idea is to make the legal product competitive with the black market," Pettinger says.

There are a bunch of advantages to that policy: Low prices boost sales and tax revenue. Discouraging illegal activity reduces law enforcement and social costs.

As the first state to decriminalize small amounts of marijuana and one of the first to legalize medical marijuana, Oregon has a strong cannabis culture and is known as a major (illicit) exporter.

In Colorado, there are three levels of tax that add up to 27.9 percent. In Washington, the rate is 37 percent, more than double Oregon's 17 percent, onto which local jurisdictions can only tack on another 3 percent.

Oregon's tax rate is simpler than Colorado's and far lower than Washington's.

"It's mainly the tax difference that makes us cheap," Whitney says.

But he warns Oregon's Dollar Tree status is in danger. States that voted to legalize pot last year will start sales with lower state taxes: California and Nevada at 15 percent, Maine and Massachusetts at 10.

Whitney says if Oregon wants to continue attracting marijuana tourists and investors to keep the industry growing, lawmakers should consider following Maine and Massachusetts.